Isolated power supply provides two types of safety:
1. Continuity of supply i.e. the isolated supply removes the possibility of earth faults therefore there is no need to blow fuse or trip a breaker to protect against earth leakage currents that could cause electrical shocks.
2. Isolated from earth allows only negligible electrical current to flow through body even an RCD will allow up to a 30mA-not sensitive enough for medical environment.